CROSS-REFERENCE TO RELATED APPLICATION
This application is related to a co-pending patent application titled "Method for Pre-Heating a Gas-Discharge Lamp", by J. K. Moriarty, Ser. No., 08/173,363, filed simultaneously with, and assigned to the same assignee, as this application.
1. Field of the Invention
This invention relates to ballasts for gas discharge lamps and the like and, more particularly, to electronic ballast circuits for driving gas-discharge lamps.
2. Description of the Prior Art
Gas discharge lighting, such as sodium vapor or fluorescence lighting, is used where the higher efficiency of gas discharge lighting over incandescent lighting is important, such as in office buildings where there may be thousands of lighting fixtures.
Each gas discharge lighting fixture or system has a ballast which controls the operation of one or more gas discharge lamp therein. The ballast serves to provide the correct voltage and current to the lamp when the fixture is first turned on and thereafter. The ballast is recognized as the component most needing improvement to increase the efficiency of gas discharge lighting.
The initial ballast designs were large transformers that operated at the power line frequency (e.g., 50 or 60 Hz) and were heavy and dissipated a lot of power. These were replaced with electronic ballasts that still relied on transformers but operated at higher frequencies (tens of KHz) to achieve better efficiencies, reduced weight and size (the transformers could be much smaller when operated at the higher frequencies). However, the transformers reduce the efficiency of the ballast. Moreover, transformer-based electronic ballast are difficult to design, relying on the electromagnetic properties of the transformer to achieve the desired voltage and current to the gas discharge lamp on startup and thereafter. Usually, these designs are a compromise between the startup and operating voltages/currents, leading to the possible reduction the life of the gas discharge lamp and/or efficiency reduction of the overall lighting system.
Thus, it is desirable to provide a ballast design that has better efficiency that prior art ballast designs.
Further, it is desirable to provide a ballast design that can be adjusted to provide the desired voltages/currents to the gas discharge lamp depending upon the level of ionization in the lamp.
Still further, it is desirable to provide an electronic ballast design with a safety feature to protect the ballast and gas discharge lamp when an overload occurs.